Photonic crystals are being actively pursued as components in optical networks, such as wavelength-division multiplexing applications. Examples of potential applications are as filters, mirrors, waveguides, and prisms. Added functionality could allow the crystals to be used in other applications such as frequency-tunable filters, optical switches, chemical and biological recognition systems, as well as other potential applications.
Three dimensional (3-D) photonic crystals have been made using a number of approaches. One common approach uses a colloidal technique that allows a distribution of spheres (e.g. microspheres—typically spheres approximately in the range of 90 nm to several microns in diameter) to settle out of solution into a bulk 3-D crystal. A similar approach uses the surface tension of a moving liquid/gas interface, created by either by pulling a substrate out of a liquid or by evaporating the liquid, to create a 3-D crystal made of up microspheres. Both techniques result in a close-packed structure of identical spheres. More complex structures are possible if differently sized spheres are used, but there is very little external control over the crystallization process and the resulting structure. The spheres can be made with a number of different materials, with polystyrene a common example, and the components are uniform in size and composition. Crystals fabricated using this technique are mechanically unstable unless a matrix such as a polymer matrix is used between the spheres to mechanically reinforce the structure.